Variable reluctance magnetic field transducer

ABSTRACT

A magnetic field transducer of the variable reluctance type provided with permanent magnetic biasing having a high degree of electromechanical coupling. The arrangement of the mechanical system is such as to isolate the dynamic changes of the air gap from hydrostatic pressures to thereby avoid deterioration of transducer calibration which would result from changes in the air gap length.

nited States Patent lPida [54] VARIABLE RELUCTANCE MAGNETIC FIELD TRANSDUCER [72] Inventor: George Pida, Arlington, Va.

[73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: Sept. 29, 1960 [21] Appl. No.: 59,453

[52] US. Cl. ..340/8 R [51] Int. Cl. ..G0lv H00 [58] Field of Search..340l8, 12, 13, 8 PC, 8 D, 8 F1,

[56] References Cited UNITED STATES PATENTS 1,321,197 11/1919 Fay ..340/8 1,896,762 2/1933 Whittle ..336/234 [451 Sept. 12, 1972 2,202,906 6/ 1940 Hawley 179/107 2,434,900 1] 1948 Black et al ..340/13 2,435,587 2/1948 Harry ..340/13 2,745,083 5/1956 Snavely ..340/8 Primary Examiner-Carl D. Quarforth Assistant ExaminerJ. M. Potenza Attorney-R. S. Sciascia, A. L. Branning and M. 1. Crane .4

[57] ABSTRACT A magnetic field transducer of the variable reluctance type provided with permanent magnetic biasing having a high degree of electromechanical coupling. The arrangement of the mechanical system is such as to isolate the dynamic changes of the air gap from hydrostatic pressures to thereby avoid deterioration of transducer calibration which would result from changes in the air gap length.

6 Claims, 1 Drawing Figure PATENTEDszmmz 3.691.515

INVENTOR GEORGE PIDA fl n/Law AGENT ATTOR .\'EY

VARIABLE RELUCTANCE MAGNETIC FIELD TRANSDUCER The invention presently to be disclosed is concerned with transducers for converting electrical signals to sound.

A large body of art exists disclosing transducers for converting electrical signals to sound waves in air. These transducers in general are not suitable for use in liquid media where the wave impedance is much higher. The problem is even greater at low frequencies where the small displacement of the transducers radiating surface must be spread over a large area in order to produce sound waves of appreciable energy.

It has been found that a suitable source for underwater use may be obtained by utilizing a radiating housing. A permanent magnet structure is attached to the housing and electromagnetics are freely suspended in close proximity to the first by means of a spring. A suitable combination of direct biasing and alternating currents applied to windings on both magnets produced a vibration of the housing. By properly adjusting the masses of the armature and stator the impedance of the transducer was matched with that of water.

Although this arrangement operated well, the large currents required to sustain the steady magnetic field reduced the efficiency of the device. The windings were also bulky and expensive. The interior bulk is important since the hollow housing must be designed to withstand the pressure of the water and large housings require stronger walls than small ones.

An object of the invention is, therefore, to provide a transducer of the type described above which is cheap, compact, and efficient.

Another object of the invention is to provide a transducer of the type described above wherein the steady biasing field is provided by a permanently magnetized source.

Other objects and attendant advantages of the invention are best understood with reference to the accompanying drawing wherein identical elements have the same numerals there is shown an isometric view of the transducer with a portion of the housing broken away and interior parts exploded to disclose inner detail.

As shown the housing 11 consists of two end portions or covers 12 and in the form of open-ended boxes and a central frame member 13 to which the covers are attached at their open ends to provide a closed interior chamber.

The left side of the figure shows shows the interior structure with the side wall of the end portions cut away and stator magnets removed. The left hand counterweight 31 and three of the springs 32 which support it can be clearly seen. The central spring supports the far edge of the weight, there being another spring at the forward end to support the near edge. The structure at the right hand end of the transducer is identical with the left hand side. The grooves 60 in the counterweights and 62 in the permanent magnet frame 13, for reception of springs 32 can readily be seen. As shown, magnets 41 are mounted on counterweights 31, wherein the magnet frame encloses the laminated permanent magnet 42.

Referring to the drawing, the relationship of the various parts can be readily discerned. The armature magnet 42 includes a plurality of rectangular plate-like permanent magnets 51 formed from magnetic material having high retentivity and retuctance. A conductive ribbon 52 is attached to the edge of each around the entire periphery. A pair of plate-like pole pieces 53 of non-retentive low reluctance magnetic material are attached to the broad faces of each of the permanent magnets.

' The electro-magnets provide a pair of closed magnetic paths through each pair of pole pieces for each permanent magnet. For convenience these magnets were formed of two cores 54 of E-shaped cross-section. U-shaped cores half this size could have been used without creating any discontinuity in the flux paths or the entire cross-section of all the cores might have been continuous. Lengthwise the cores may be built up in thin layers. While four permanent magnets are shown, any number may be used.

Insulated magnet wire 55 is placed in the grooves of the magnets 41 so that a current passing therethrough will produce a continuous flux path around each magnet 51, the flux in adjacent pole pieces 53 being in the same direction. The magnet wire is interconnected using the passageway 56 to connect the magnets at opposite end of the transducer. The windings are connected to a waterproof cable (not shown) which is sealed through an aperture in one of the covers.

This transducer does not require any new or unusual materials in its manufacture. The housing including the covers 12 and magnet frame 13, for example, may be from any suitable metal or alloy. The frame must be nonmagnetic to avoid a short circuit of the magnetic circuits. Since the housing required a larger volume of material than the electromagnetic structure and a mass ratio of unity was desired in this particular transducer, a low density aluminum alloy was chosen.

The remainder of the permanent magnet structure also includes well known materials. The permanent magnets were made from Alnico V" a well known alloy of 8 percent aluminum, 14 percent nickel, 24 percent cobalt, 3 percent copper, and the remainder iron. The ribbon or strip surrounding each magnet 52 was made of copper. The pole pieces 53 were made from silicon steel strips, cold rolled to provide grain orientation.

In the electromagnetic structure, spring steel was used to make the springs 32. The counterweights 31 were made of steel. The electromagnetic magnet core 41 was fabricated of the same material as the armature pole. The windings were formed from a well known vinyl plastic covered magnet wire.

In making the transducer, the permanent magnets 5 l are first shaped and edged with copper. The copper may be plated or, as was done in the present case, formed from a strip soldered at its ends and then cemented with an epoxy resin to the magnet. The pole pieces are then constructed by cementing together the broad faces of a plurality of thin strips of transformer steel, as for example that mentioned above. The longer exposed edges of the strips are then cemented to a broad face of the magnet.

Magnet 41 are formed by cementing enough E- shaped stampings from a silicon steel sheet, for example, to form a core of the proper length and cementing two or more of these cores together, as shown. The windings are first wound on a mandrel, impregnated with cement and then cemented in the slots provided.

Before the permanent magnets are cemented in the frame 13, a keeper bar is placed across the ends of the pole pieces which are to be presented at the air-gap. The magnets are then magnetized to saturation by placing the broad faces thereof in contact with the pole pieces of a large induction magnet and applying sufficient magnetomotive force.

Magnet 41 are carefully installed so as to maintain as strong a flux as possible in the magnetic circuit. To do this one or more shims of brass or other suitable nonmagnetic material, having substantially the same thickness as will the air-gap in the finished transducer, is placed over the poles of the cores 54. The shims are best made larger than the end area of the magnet poles so that an end portion will protrude beyond the poles along one edge. This end portion can then be easily grasped for removal. The electromagnetic structure is then employed to displace the keeper bars by sliding the shimmed surface over the pole pieces of the armature magnets. Thus a continuous flux path is provided by either the keeper bars or the cores at all times.

The unit is further assembled by cementing the counterweights in place, sliding three of the springs into the grooves provided in the frame and counterweights, cement having been applied to all areas of contact, and removing the shims from the air-gap. With the shims removed, the fourth spring is similarly inserted. The various cementing steps may be hastened by baking at appropriate stages of construction. Epoxy resin cement is used throughout. The corners of the springs and counter weights may be chamfered to facilitate their assembly.

The structure is completed by effecting the necessary electrical connections and cementing the covers in place. The electrical connections merely involve passing one or both the ends of one winding through the passageway and soldering the ends of both windings to the waterproof cable which is properly sealed through the wall of one cover.

in operation an alternating current passes through the windings producing fluxes which alternately aid and buck the constant fluxes across each gap. The coils at each end are connected, so that the alternating flux across one gap is aiding the constant flux while the two are opposing in the remaining gap. This push-pull action provides a linear coupling between the electrical and mechanical systems and places a balanced load on the electrical signal source. Connecting the electromagnets at each end to the housing and freeing the central permanent magnet was considered, but hydrostatic forces on the housing have a tendency to alter the air-gap in this arrangement.

The exact operating characteristics vary with the size of the transducer. The overall dimensions of the one embodiment were 5- 7: by 5-56 by 9 inches and weighed 26.5 lbs. The permanent magnets were 2 inches square and one-fourth inch thick. The pole pieces were three sixteenth inches thick. The housing walls were one-half inch thick. An air-gap of 2.5 milli-inches was provided. Other relative sizes may be adduced from the drawing. With a unity ratio between radiating and nonradiating masses the structure resonated at 412 cycles with a Q of 49.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: l. A transducer for underwater sound comprising: a sealed rigid housing; an armature magnet having a plurality of pole pieces providing dc. bias located within said housing, with said armature magnet being centrally attached thereby dividing said housing into two distinct chambers; electromagnet means also having a number of pole pieces suspended within each of said chambers, with said electromagnet means providing a.c. bias;

spring means for suspending said electromagnets adjacent said armature magnets creating an air gap between said electromagnets and armature magnet whereby upon applying a.c. exciting current to said electromagnets the air gaps are alternately reduced and increased during the ac. half cycle creating a piston-like action at the ends of said housing providing a linear coupling and a balanced load.

2. The transducer according to claim 1 wherein said armature magnets include a highly conductive material on the edge of each pole piece thereby isolating said permanent magnets from said a.c. exciting current preventing permanent magnet demagnetization wherein added stability in said transducer is maintained.

3. The transducer according to claim 2 wherein the pole pieces of said armature magnet and said electromagnet are adjacent and opposite each other thereby providing a closed magnetic path through each pair of pole pieces on said magnets.

4. In combination in an electroacoustic transducer, a rigid plate member having at least one face which contains a plane area, a first magnetically-conducting element rigidly attached to said plane area, a second magnetically-conducting element mounted in operable relationship with said first magnetically-conducting element, spring members separating said first and said second magnetically-conducting elements, electrical coil means mounted in operable relationship with both said magnetically-conducting elements, electrical terminal means connected to said coil means, and housing means comprising a hollow shell-like structure mechanically attached to the peripheral portion of said rigid plate member, and totally enclosing said magnetically-conductive elements and associated electrical coil means.

5. In an electroacoustic transducer for operation in deep water, a rigid plate member, first magneticallyconducting means rigidly attached to one surface of said rigid plate member, second magnetically-conducting means, spring means mounting said second magnetically conducting means in operable relationship to said first magnetically-conducting means with an air gap therebetween, electrical conducting coil means mounted in operable relationship with said first and second magnetically-conducting means for effecting vibratory movement of said second magnetically-conducting means toward and away from said one surface of said rigid plate member, and housing means comprising a hollow shell-like structure mechanically at- 6. An electroacoustical transducer as set forth in claim 5, wherein said shell-like housing structure comprises a pair of cup-like sections having brim portions with said peripheral portion of said rigid plate member being mechanically held between said rim portions. 

2. The transducer according to claim 1 wherein said armature magnets include a highly conductive material on the edge of each pole piece thereby isolating said permanent magnets from said a.c. exciting current preventing permanent magnet demagnetization wherein added stability in said transducer is maintained.
 3. The transducer according to claim 2 wherein the pole pieces of said armature magnet and said electromagnet are adjacent and opposite each other thereby providing a closed magnetic path through each pair of pole pieces on said magnets.
 4. In combination in an electroacoustic transducer, a rigid plate member having at least one face which contains a plane area, a first magnetically-conducting element rigidly attached to said plane area, a second magnetically-conducting element mounted in operable relationship with said first magnetically-conducting element, spring members separating said first and said second magnetically-conducting elements, electrical coil means mounted in operable relationship with both said magnetically-conducting elements, electrical terminal means connected to said coil means, and housing means comprising a hollow shell-like structure mechanically attached to the peripheral portion of said rigid plate member, and totally enclosing said magnetically-conductive elements and associated electrical coil means.
 5. In an electroacoustic transducer for operation in deep water, a rigid plate member, first magnetically-conducting means rigidly attached to one surface of said rigid plate member, second magnetically-conducting means, spring means mounting said second magnetically conducting means in operable relationship to said first magnetically-conducting means with an air gap therebetween, electrical conducting coil means mounted in operable relationship with said first and second magnetically-conducting means for effecting vibratory movement of said second magnetically-conducting means toward and away from said one surface of said rigid plate member, and housing means comprising a hollow shell-like structure mechanically attached to the peripheral portion of said rigid plate member and totally enclosing said magnetically-conducting means and said electrical conducting coil means, said shell-like structure being deformable under high pressure in deep water without affecting said air gap.
 6. An electroacoustical transducer as set forth in claim 5, wherein said shell-like housing structure comprises a pair of cup-like sections having brim portions with said peripheral portion of said rigid plate member being mechanically held between said rim portions. 